The purpose of the proposed study is to examine factors associated with transmission of sexually transmitted diseases (STDs) in a high-risk population: street youth. Preliminary data suggest that street youth may be functioning as a core population, maintaining some STDs at endemic levels. In order to determine if this hypothesis is correct, and if so, to what extent street youth function as an STD reservoir, we propose to examine a street youth population for STDs and to assess both the behaviors and the antecedent constructs related to transmission of STDs. Using both cross-sectional and longitudinal data, we propose to: (a) test the applicability of epidemiological models of STD prevalence trends and infection acquisition, in the prediction of STD prevalence and incidence in a street youth population, by measuring these trends and applying the empirically-determined values, (b) examine the concurrent and prospective relationships between factors relevant to the epidemiological model variables (i.e., partner exchange, care-seeking, and STD transmissibility) and constructs related to personal, social, and environmental influences, and (c) use social network analysis to characterize the social connections by which STDs are spread within this population and to other populations as well. Three waves, with 200 subjects in each, will be recruited at approximately ten month intervals. Each wave will be assessed at baseline (T1), three months later (T2) and six months later (T3). The T1 assessments will provide STD prevalence data and cross-sectional data for other measure while the T2 and T3 assessments will provide STD incidence data and longitudinal data on personal, social, and environmental constructs, as well as information regarding the stability of social networks.